1. Technical Field
This invention relates to a burner for use in the manufacture of synthetic quartz glass ingots useful as the stock material for excimer laser synthetic quartz glass optical members and large-diameter synthetic quartz glass ingots useful as the stock material for liquid crystal-related large-size photomask substrates. More particularly, it relates to a burner for use in the manufacture of synthetic quartz glass ingots having optical-grade high homogeneity and a minimal change of light transmittance and useful as optical members such as lenses, prisms, mirrors, windows and photomask substrates in excimer laser systems, especially ArF excimer laser systems. The invention also relates to a method for the manufacture of synthetic quartz glass ingots.
2. Background Art
To meet the recent trend of LSI toward higher integration, the photolithography of defining an integrated circuit pattern on a wafer requires an image exposure technique on the order of submicron units. For finer line width patterning, efforts have been made to reduce the wavelength of a light source of the exposure system. In the lithography, a KrF excimer laser (wavelength 248 nm) took over the prior art i-line (wavelength 365 nm) as the mainstream light source in steppers; and the practical use of an ArF excimer laser (wavelength 193 nm) has recently started. Then, the lens for use in steppers is required to have homogeneity, improved UV transmission, and resistance to UV irradiation.
In order to avoid contamination with metal impurities which cause UV absorption, the synthesis of quartz glass is generally carried out by introducing the vapor of a high purity silicon compound such as silicon tetrachloride directly into an oxyhydrogen flame. Flame hydrolysis takes place to form silica fines, which are directly deposited on a rotating heat-resistant substrate such as quartz glass while being melted and vitrified thereon. In this way, transparent synthetic quartz glass is produced.
The transparent synthetic quartz glass thus produced exhibits satisfactory light transmittance in the short-wavelength range down to about 190 nm in the LSI field. It has been utilized as a material capable of transmitting UV laser light, specifically i-line and excimer laser light such as KrF (248 nm), XeCl (308 nm), XeBr (282 nm), XeF (351 and 353 nm) and ArF (193 nm), and the four-fold harmonic wave (250 nm) of YAG.
The absorption of light in the UV region that is newly created by irradiating synthetic quartz glass with UV light having great energy as emitted by an excimer laser is deemed to be due to the paramagnetic defects formed through photo-reaction from intrinsic defects in synthetic quartz glass. Many light absorption bands due to such paramagnetic defects have been identified by ESR spectroscopy, for example, as E′ center (Si.) and NBOHC (Si—O.).
The paramagnetic defects generally have an optical absorption band. When UV light is irradiated to quartz glass, the problematic absorption bands in the UV region due to paramagnetic defects in quartz glass are, for example, at 215 nm due to E′ center (Si.) and 260 nm, which has not been accurately identified. These absorption bands are relatively broad and sometimes entail strong absorption. This is a serious problem when quartz glass is used as a transmissive material for ArF and KrF excimer lasers.
Intrinsic defects in synthetic quartz glass which cause paramagnetic defects arise from structures other than SiO2 such as Si—OH and Si—Cl and oxygen-depleted or enriched structures such as Si—Si and Si—O—O—Si. As the approach for suppressing paramagnetic defects, it is proposed in JP-A 6-199532 to use a chlorine-free alkoxysilane such as tetramethoxysilane as the silane compound for preventing Si—Cl, one of paramagnetic defects, from being incorporated in glass.
It is also known that if hydrogen molecules are present in quartz glass in a concentration above a certain level, few defects of E′ center (Si.) which are oxygen defects are formed, leading to improved durability to laser damage. Since ArF excimer laser light causes several times serious damages to quartz glass as compared with KrF excimer laser light, the quartz glass for ArF laser application must have several times higher a hydrogen molecule concentration than the quartz glass for KrF laser application.
It is proposed in JP-A 6-305736 to control the hydrogen molecule concentration in synthetic quartz glass. Depending on the energy using conditions of an ArF laser, the hydrogen molecular concentration in glass is adjusted.
Now that the efforts to reduce the wavelength of light source have reached excimer laser light having extremely greater energy than the traditional i-line light, active research works have been made on the laser durability of glass.
Exposure apparatus using such shorter wavelength light include many optical parts such as lenses, windows, prisms, and photomask-forming quartz glass substrates. With respect to projection lens materials among these optical parts used in exposure apparatus, the recent progress is toward a higher NA, the diameter of lens is annually increasing, and the optical homogeneity of lens material is required to be of higher precision. Especially for the ArF excimer laser, it is required that the initial transmittance of quartz glass, specifically the transmittance at wavelength 193.4 nm over the entire surface of an optical member be close to the theoretical value, the theoretical value at wavelength 193.4 nm being computed to be 99.85% by taking into account multiple reflection. Since the optical system in the exposure apparatus is composed of several to several tens of lenses, it is important that setting an initial transmittance of quartz glass even a little higher restrains the absorption of optical energy within the bulk of quartz glass, thereby minimizing a possibility that the light energy once absorbed is converted to thermal energy to incur a change of density and in some cases, a change of refractive index. In addition to the essential uniformity of refractive index, a reduction of birefringence becomes a crucial problem.
As stated above, in order to avoid contamination with metal impurities which cause UV absorption, the synthesis of quartz glass is generally carried out by introducing the vapor of a high purity organosilicon compound such as silicon tetrachloride directly into an oxyhydrogen flame. Flame hydrolysis takes place to form silica fines, which are directly deposited on a rotating heat-resistant substrate such as quartz glass and melted and vitrified thereon to form transparent synthetic quartz glass. The synthetic quartz glass ingot thus produced is sliced perpendicular to its growth direction whereupon a distribution of transmittance at wavelength 193.4 nm is determined in a plane of the growth direction. Then, the slice has an in-plane distribution, typically with the tendency that transmittance decreases from the center to the periphery. If the value required for the initial transmittance is at least 99.7% as an internal transmittance, for example, an effective portion of the synthetic quartz glass ingot that can be utilized, generally known as percent yield, is determined by this value.
Apart from the LSI application, large-size quartz glass substrates for photomasks are now used in the liquid crystal display (LCD) application. It is required to form synthetic quartz glass ingots for use as the stock material therefor to larger diameters, particularly when the percent yield of the manufacturing process of large-size glass substrates is considered. While the mainstream of conventional synthetic quartz glass substrates for IC use is of 6 inch square size, large-size glass substrates have already been required to have one side of 1 meter or longer. In fabricating large-size quartz glass substrates, as opposed to the synthetic quartz glass ingot for IC use which must have a diameter of about 100 to 140 mm, for example, an ingot which is of a conventional ingot diameter must be increased in length in order to insure a certain product weight. Shaping must be repeated many times until the size is tailored to a desired profile. The situation is detrimental in production yield and efficiency.